1. Field of the Invention
The present invention relates to an Ni—Fe based forging superalloy which is excellent in high-temperature strength and high-temperature ductility and a method of manufacturing the same, and also to a steam turbine rotor formed of the Ni—Fe based superalloy forging material.
2. Description of the Related Art
In order to improve power generation efficiency of a steam turbine power plant, an elevating of the main steam temperature is effective. At present, steam turbine generator plants of the 600° C. class, in which main steam temperature exceeds 600° C., are commercially employed and for the aim of further improvements to the power generation efficiency, development of the steam turbines having a main steam temperature of the 650° C. class and a main steam temperature of the 700° C. class is proceeding.
Iron and steel materials formerly used as the rotor materials are not applicable to the steam turbine of the 700° C. class, in which the main steam temperature is 700° C. or more, since such iron and steel materials have a durable temperature of about 650° C. Accordingly, it is necessary to use an Ni based superalloy having a higher durable temperature than that of the iron and steel materials, as the rotor material. Such an Ni based superalloy has higher strength than that of iron and steel materials. However, the Ni based superalloy is expensive and also is hard to produce as large forged products.
Ni based superalloys relatively facilitating the manufacturing of large forged products include A286 type, IN706 type and IN718 type Ni—Fe based superalloys. These Ni—Fe based superalloys are employed in gas turbine disks, generator rotors, etc.
The A286 type alloy is advantageous in terms of cost, since it has much Fe. However, the A286 type alloy is not suitable for the material of the steam turbine rotor having a main steam temperature of 700° C. or more, since it has weak strength. Further, the IN706 type alloy has better strength. However, the IN706 type alloy is difficult to manufacture as forged products having a weight of more than 10 tons, since the IN706 type alloy has much more content of Nb being a segregation element. Furthermore, the IN718 type alloy has better high-temperature strength than that of the IN706 type alloy. However, the IN718 type alloy has much more content of segregation elements such as Nb and Mo, and thus causes segregation easier compared with the IN706 type alloy. Therefore, it is also difficult to manufacture a steam turbine rotor having a weight of more than 10 tons based on the IN718 type alloy. Also, as used herein, the “weight of the rotor” means the weight of a finished product of a single rotor.
JP-A-2005-2929 (Abstract) discloses that a steam turbine rotor is manufactured by using Ni—Fe based superalloy forging materials that are successfully obtained by decreasing the amount of Nb compared with IN706 type alloy or IN718 type alloy and further adding Al to it to stabilize a γ′ phase and to suppress the precipitation of harmful phases (i.e., η, δ, α phases) incurring embrittlement. Further, it discloses that a steam turbine rotor of a 10 ton class can be manufactured.
Furthermore, in Superalloy 718, 625, 706 And Various Derivatives TMS, 2005, p. 77 (non-patent document 1), mechanical properties of an Ni—Fe based superalloy which was subject to solution treatment and aging treatment are shown.